This invention relates to an instrument for measuring the difference between two physical quantities using electrical sensing transducers.
More particularly, this invention relates to the use of transducers which do not generate voltage or current by themselves, but transducers which require the application of an external source of electrical power, and the voltage developed across or the current flowing through the transducers is detected and used as a measure of the quantity measured, such as temperature, pressure or some other physical quantity which affects the resistance, reactance or other characteristic of the transducer. Generally, such transducers are non-linear devices.
The most common means of measuring the differential between two quantities is to use a resistive or reactive electrical bridge where the transducers form two legs of that bridge. By carefully matching the fixed components, the bridge output can be made to represent closely the difference between the quantities sensed by the transducers; and the output can represent, in some cases, the linear equivalent of the difference detected by the transducers over some narrow range. In some devices, feedback from the output of the bridge is used to modify the bridge exitation current to provide linearization. The effects of lead wire resistance are reduced in some instruments by the use of compensating loops in the transducer connections.
Although the use of an electrical bridge minimizes error due to inbalance in the measurement circuitry, there are limits in the accuracy and in the range over which linearization of the instrument can be maintained. All of these instruments are susceptible of lead wire resistance, as none permit true four-wire measurement of the transducer's outputs. Direct current energized bridges will produce undesirable outputs due to thermocouple effects in the transducer connections. All require both transducers to be energized simultaneously, and this is a serious limitation in power-limited systems.
Another common means of measuring the difference between two transducers includes amplifying, scaling and linearizing the output signal of each transducer separately and then electrically subtracting one amplified signal from the other. Such devices may be designed to provide any degree of linearization required, and may use true four-wire measurements to eliminate errors due to lead wire resistance. These systems require careful matching and close temperature tracking of the two amplifying, scaling and linearizing means in order to avoid large errors in the measured difference signals. Also, direct current energized instruments remain sensitive to thermocouple effects, and all systems require that both transducers be energized simultaneously.
In the measurement of low level direct current signals, such as those produced by the transducers described above, chopper stabilized amplifiers may be used to minimize errors caused by direct current offset voltages and other currents found in the amplifiers; however, chopper stabilization does not reduce sensitivity to thermocouple effects generated in the connection between the transducers and the connecting cables.